I have a letter glued onto plywood and want to remove it. What is the best way?. I haven’t tried anything yet because it is an historical document.
The method of removal will depend very strongly on what kind of glue was used -- and it's very likely you don't/can't know that. For genuinely historical paper in similar straits, it's been done to flake the paper off and reassembled the pieces, face down, on a conservation material using a reversible adhesive, then apply a reinforcement on the back using a more permanent adhesive before releasing the now-whole document from the face layer.
This, however, is well beyond the purview of amateurs. If your document has importance, it's better not to do additional damage, than to risk furthe destruction trying to "rescue" or "repair" its condition.